The present invention relates to a method for generating and transmitting xe2x80x9csilencexe2x80x9d for use in a Pulse Code Modulation (PCM) data communication system. The communication system of particular interest herein uses the public digital telephone network (DTN) to transmit data directly from a digital source to a remote unit, where the remote unit is connected to the DTN either digitally or via an analog local loop. Within this PCM data communication system it is desirable to utilize a xe2x80x9czeroxe2x80x9d signal. The signaling method described herein a method of forming and transmitting a zero signal, and is compatible with both the xcexc-Law and A-Law PCM coding, and minimizes the impact of coding differences between the two PCM coding systems.
Presently, typical modems used to communicate over the public telephone system represent binary data by an analog waveform that is modulated in response to the binary data. As an example, one such standard for modem communications is detailed in the International Telecommunication Union, Telecommunication Standardization Sector (xe2x80x9cITU-Txe2x80x9d) Recommendation V.34 (1994). The waveform is in turn analyzed at a receiving modem to recover the binary data. For modem signals transmitted over the public telephone system, the analog waveforms are treated by central office facilities in the same manner as if the waveforms were analog voice signals. In other words, the waveforms are digitized into eight bit octets by an analog to digital converter (ADC) codec at the central office, and the octets are transmitted in digital format between central offices until they are converted back to an analog signal by a digital to analog converter (DAC) codec at the central office that is connected to the receiving subscriber loop. The public switched telephone network has operated in this manner for many years.
The data rate attainable by a modem operating in such an environment is limited by numerous factors including, in particular, the codec sample rate and the number and spacing of quantization levels of the codec converters at the central office switches. The effect on an analog signal associated with sampling the signal amplitude and representing the sample by one of a finite number of discrete (digital) values is generally referred to as quantization noise. Most telephone switches utilize voice codecs that perform nonlinear A/D and D/A conversions known as xcexc-law or A-law conversion. In these conversion formats, the 8-bit codec codewords, also referred to as octets, represent analog voltages that are nonlinearly spaced. This type of conversion performs well for voice signals intended for a human listener (especially when transmitted over a noisy line), but have a negative impact on modulated analog waveforms associated with modems. Specifically, codecs that adhere to these standard nonlinear conversion formats use nonlinearly spaced quantization levels, and have the effect of increasing quantization noise which is detrimental to modulated analog waveforms.
Until recently, it was thought that the maximum attainable data rate for signals passing through the DTN was limited by the quantization noise associated with the codecs. However, it has been recognized that a data distribution system can overcome certain aspects of the aforesaid limitations by providing a digital data source connected directly to the DTN, without any intervening ADC or DAC. In such a system, the telephone network routes digital signals from the data source to the client""s local subscriber loop without any intermediary analog facilities, such that the only analog portion of the link from server to client is the client""s local loop (plus the associated analog electronics at both ends of the loop). The only DAC in the transmission path is the one at the Telephone Company""s end of the client""s subscriber loop. In such a system digital data can be converted into PCM codes, and fed to the DTN as 8-bit bytes (octets) at the network""s clock rate of 8 kHz. At the distant end, the DTN""s DAC converts each byte to one of 255 analog voltage levels in a system utilizing xcexc-law encoding (or 256 levels in an A-law system), which is sent over the client""s subscriber loop and received by a subscriber device (i.e., a modem) at the client""s location.
FIG. 1 shows a block diagram of a PCM data distribution system. The system includes a data source 10, or server, having a direct digital connection 30 to a digital telephone network (DTN) 20. A client 40 is connected to the DTN 30 by a subscriber loop 50 that is typically a two-wire, or twisted-pair, cable. The DTN routes digital signals from the data source 10 to the client""s local subscriber loop without any intermediary analog facilities such that the only analog portion of the link from the server to the client is the client""s local loop 50. The analog portion thus includes the channel characteristics of the local loop transmission line plus the associated analog electronics at both ends of the line. This typically includes a subscriber line interface card at the central office that includes a codec, as well as circuitry used to generate and interpret call progress signals (ring voltage, on-hook and off-hook detection, etc.). The only D/A converter in the transmission path from the server to the client is the one at the DTN end of the client""s subscriber loop. It is understood that the client-side, or subscriber-side, equipment may incorporate an A/D and D/A for its internal signal processing, as is typical of present day modem devices. For the reverse channel, the only A/D converter in the path from the client to the server is also at the Telephone Company""s end of the client""s subscriber loop.
An alternative system is one where connection 50, like connection 30, is a digital connection to subscriber unit 40. In such a system there are preferably no analog transmission links, that is, the digital PCM codewords are not converted to one of a plurality of an analog voltage levels, but transmitted directly to unit 40 in binary form. Of course, at the physical layer, the transmission of the binary signals or codewords is performed by transmitting voltage levels representing logic signals having values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d.
The conversion from octet to analog voltage is well known, and is based on a system called xcexc-law coding in North America. In Europe, a format known as A-law coding is used. Theoretically, there are 256 points represented by the 256 possible octets, or xcexc-law/A-law codewords. FIGS. 2A-C show the positive values of the xcexc-law and A-law codewords. There are one hundred twenty eight values, and a total of two hundred fifty six values including the negatives. The codewords are given in hexadecimal format, and are ordered according to the corresponding analog voltage level. Note that the analog level is calibrated in integers from 0 to 32,124. These numbers correspond to a linear 16-bit A/D converter. As is known to those of ordinary skill in the art, the sixteenth bit is a sign bit which provides integers from 0 to xe2x88x9232124 which correspond to the first 128 octets (hexadecimal 0 to 7F), not shown in FIG. 2 or 3.
FIG. 3 plots the xcexc-law codewords versus the analog voltage level. FIG. 3 can be viewed as a conversion between the logarithmic binary data and the corresponding linear 16-bit binary data. It can also be seen in FIG. 3 that the logarithmic function of the standard xcexc-law conversion format is approximated by a series of 8 linear segments.
The format of the xcexc-law codewords is shown in FIG. 4, where the most significant bit b7 indicates the sign, the three bits b6-b4 represent the linear segment, and the four bits, b0-b3 indicate the step along the particular linear segment. These points are symmetric about zero; i.e., there are 128 positive and 128 negative levels, including two encodings of zero. Since there are 254 points not including zero, the maximum number of bits that can be sent per signaling interval in a system having an analog link in the channel (symbol) is just under 8 bits. In an all digital system, all 256 octets may be utilized, resulting in eight bits per signaling interval. Other factors, such as noise, digital attenuation (pads), channel distortion introduced by the subscriber loop, and the crowding of points at the smaller voltage amplitudes and the associated difficulty in distinguishing between them at the decoder/receiver, may reduce the maximum attainable bit rate. Note that the A-law format, however, has no codewords corresponding to an analog voltage level of zero volts. As seen in FIG. 2A, the codeword D5 represents a linear value of 8, the smallest linear value. The corresponding negative codeword having a linear value of xe2x88x928 is obtained by changing the sign bit, and is hexadecimal 55.
In the system shown in FIG. 1, digital data can be input to the DTN as 8-bit bytes (octets) at the DTN""s clock rate of 8 kHz. This is commonly referred to as a DS-0 signal format. In the system having an analog subscriber loop, the DTN""s interface to the subscriber loop includes a codec that converts each byte to one of 256 analog voltage levels (although two of these are zero volts in the xcexc-law system). These voltage levels are sent over the client""s subscriber loop and received by a decoder at the client""s location. As shown in FIG. 3, the analog voltages, or points, corresponding to the quantization levels are non-uniformly spaced and follow a generally logarithmic curve. As can be seen in FIG. 3, the increment in the analog voltage levels produced from one codeword to the next is not linear, but depends on the mapping as shown.
Certain network connections utilize a supervisory signaling technique called Robbed Bit Signaling (RBS). On RBS links, the least significant bit (LSB) of the PCM code is usurped, or xe2x80x9crobbedxe2x80x9d, by the network periodically and used to convey control information. The PCM codes from different channels are grouped together and multiplexed into frames, typically 24 DS-0 channels plus a framing bit, to create a DS-1 signal that may be sent over a T-1 carrier system. In typical robbed bit signaling, the T-1 carrier system uses the LSB of every channel, every sixth frame, for sending control and status information between network equipment. Thus, each DS-0 user loses the use of the LSB every sixth octet (once per every sixth DS-1 frame). Ordinarily, these channels are used for voice communications, and the bit robbing merely increases quantization noise of the effected time slots. The effect of robbed bit signaling on voice quality is barely perceptible to the human ear.
To control power levels, some networks impose digital attenuators. Unlike analog attenuators, a network digital attenuator (NDA) is not linear. Because there are a finite number of digital levels to choose from, the NDA will be unable to divide each codeword exactly. This causes distortion of the analog level ultimately transmitted by the DAC over the subscriber loop. For example, if the NDA is designed to reduce voltage levels by xc2xd (6 dB), then PCM code 130 will attenuate to code 146, and the corresponding analog levels will not be related exactly by a factor of xc2xd. Also, NDAs can produce code ambiguities, which happens more frequently with codes corresponding to small absolute linear values. Specifically, more than one codeword may be reduced to the same attenuated codeword, resulting in an ambiguity. Another type of ambiguity may be introduced when low-amplitude codewords of positive and negative values pass through an NDA and are transformed to a codeword having the smallest magnitude, but perhaps having a positive value. That is, e.g., in a xcexc-law format, both negative and positive zeroes (hex values 7F and FF) are converted to the same codeword.
At certain points in data transmission, it is desirable for the devices used at each end of the above-described data transmission system to transmit silence, or the equivalent of an analog zero voltage level. The silence is often transmitted before the nature of the channel is fully determined. That is, the device may attempt to transmit a zero signal over a channel that may include NDAs or RBS signaling, or may be fully digital end-to-end, or may have analog portions of the channel. These impairments will interfere with the transmission of the desired zero signal.
A method of transmitting a quite, or zero, signal in a PCM communication system is provided. The zero signal is specified universally in ordered set terms for either xcexc-Law or A-Law PCM systems, which has minimal energy in the A-Law system, and contains no energy within the xcexc-Law system. The signal is preferably specified as a repetition of six intervals (or multiple thereof) because the DTN can modify PCM codes on a six interval period by the robbed-bit signaling mechanism. The zero signal may be used to detect network elements that produce single-signed zero outputs from zero inputs of either sign. In addition, by examination of the zero signal the receiver may determine whether the channel includes an analog link or connection.